In the field of power applications, it is common to use linear power amplifiers. While linear power amplifiers operate in a generally linear manner they are known to have a disadvantage of achieving low efficiency. In high power applications, such as the transmit chain of a wireless base station, this requires expensive power devices which consume a considerable quantity of power and require large heatsinks and associated cooling equipment to maintain them at an optimum operating state.
In order to increase the efficiency of a linear power amplifier, it is known to modulate the power supply to the linear power amplifier with the envelope of the signal which is to be amplified. The power supply can be a Pulse Width Modulated (PWM) power supply in which a power switching device, such as a power transistor, is turned on and off at a high frequency, with the width of the ‘on’ periods varying in sympathy with the amplitude of an input signal. The resulting train of pulses is smoothed by a low pass filter to deliver the amplifier with a power supply which tracks the envelope of the input signal.
The PWM power supply can have a single phase or, more usually, can have multiple phases, with the contributions of individual phases summing to provide an overall output. Multi-phase PWM power supplies have an advantage over single phase PWM supplies in that they can deliver better resolution and increased current. It should be noted that the term ‘phase’ relates to apparatus for sampling an input signal and operating a switching device. Thus, a multi-phase PWM supply has multiple switching devices which operate at times that are offset from one another.
Switching devices have a finite frequency range over which they can be reliably operated, and begin to work in a non-ideal manner when operated towards the extremes of their operating range. The frequency at which the switching devices in the power supply operate must be greater than the highest frequency in the input signal, to avoid aliasing distortion effects. For an input signal having a wide bandwidth, this requirement forces the switching device to operate in the region towards the upper boundary of its operating range, which incurs switching losses and begins to cause non-ideal behaviour.
As an example, the base stations in a third generation, four channel Universal Mobile Telecommunications System (UMTS) are required to transmit and receive signals having a bandwidth of around 20 MHz, which requires switching devices in the PWM power supply to operate at rates well in excess of this. These rates are at the upper limits of present switching technology. During signal troughs the current demand is low which reduces the negative slew rate capability of the supply and can cause a conventional PWM converter to go into discontinuous operation.
With increasing competition among operators, there is a desire to reduce costs. With power costs being one of the most significant operating costs for base stations, there is a desire to reduce these.
Accordingly, the present invention seeks to provide a power supply which is more efficient in operation.